Fiber-reinforced composite materials, which are made up of reinforcing fibers and matrix resins, are lightweight and have excellent mechanical properties. As such, these composite materials have been widely used in a variety of structural and non-structural applications such as aerospace, automotive, infra-structure repair, marine, military, and sporting goods or other consumer products that must have sufficient toughness and impact resistance to endure many years of harsh service.
Various methods or techniques such as prepreg, hand lay-up, filament winding, pull-trusion, RTM and RI, have been used to produce fiber-reinforced composite materials. Of these methods or techniques, the RTM method, in which a preform made up of reinforcing fibers is placed in a mold, a resin poured therein to impregnate the preform, and the impregnated preform cured to produce a molded product, offers the advantage that a large component having a complicated shape can be molded in a short period of time.
Epoxy resins, and to a lesser extent, unsaturated polyester resins, vinyl ester resins, phenol resins, and polyimide resins, have been employed as matrix resins in fiber-reinforced composite materials. The use of composite materials having polyimide resin matrices is increasing, however, where these materials are now recognized as preferred structural materials in aerospace applications, because of their lightweight and load-bearing characteristics and their oxidative stability at elevated temperatures.
Fiber-reinforced composite materials that use polyimide resins as the matrix resin are generally prepared using prepreg methods. Current technologies for making prepreg and composites from polyimides utilize solutions from the poly(amide) acids of these resins. Poly(amide) acid solutions are processed into prepreg with various reinforcing fibers. These poly(amide) acid solutions are of low solids contents and high viscosity. Therefore, the processing of these types of solutions requires overcoming significant problems such as solvent management and good fiber wet out from the high viscosity solutions. The resultant prepreg typically requires residual solvent contents of 20 to 25% by weight (approximately 2–3% water from thermal imidization reaction) for adequate tack and drape. This residual solvent must then be removed during the composite cure cycle. This material is hand-laid into composites which makes working with this type of material very labor intensive and costly.
The drawbacks inherent in prepreg methods have prompted the development of polyimide resins suitable for processing by RTM and RI methods. The developed resins, however, require relatively high processing and cure temperatures of greater than 250° C., which severely limits their industrial utility due to the need for employing specialized high temperature molding equipment.
For example, Jim M. Criss et al., Resin Transfer Molding and Resin Infusion Fabrication of High Temperature Composites, Proceedings of the 46th International SAMPE Symposium, Vol. 46 (2001), discloses two phenylethynyl containing imide oligomers that are processable by RTM and RI methods. The two oligomers, which are designated PETI-RTM and RFI, reportedly display low and stable melt viscosities at temperatures of 250 to 290° C. PETI-RTM is defined as BPDA//25 mole % 3,4′-ODA/75 mole % APB//PEPA, having a Mn=750 g/mole, while PETI-RFI is defined as BPDA//25 mole % 3,4′-ODA/75 mole % APB//PEPA, having a Mn=1250 g/mole. Composites are prepared by RTM using the PETI-RTM and PETI-RFI oligomers and by RI using the PETI-RFI oligomer, at processing temperatures of greater than 250° C. The injection temperature used was in the range of 260 to 288° C., while the cure temperature was 371° C.
U.S. Pat. No. 5,965,687 to Brian J. Jensen discloses mixtures of polymeric materials consisting of branched, star-shaped and linear polyimides, which are synthesized by using a small yet critical amount of a trifunctional monomer (e.g., a slow-reacting triamine such as triamino pyrimidine or melamine), along with the conventional difunctional monomers in the polymerization. These mixtures reportedly have lower melt viscosities than linear polymers at the same molecular weight thereby allowing for processing via RTM or RFI at lower pressures and temperatures with techniques such as autoclave processing. See Col. 2, lines 51 to 55, of U.S. Pat. No. 5,965,687. Processing temperatures of greater than 250° C. are still required, however, thereby limiting the industrial utility of these mixtures.
U.S. Pat. No. 6,124,035 to Connell et al. discloses high temperature transfer molding resins prepared from aromatic diamines containing phenylethynyl groups and various ratios of phthalic anhydride (PA) and 4-phenylethynyl phthalic anhydride (PEPA). These resins reportedly have, among other things, relatively low melting temperatures (˜182° C.), low melt viscosities (<1 poise at ˜270° C.), and excellent melt stabilities (>2 hours at 250˜280° C.). As noted above, however, the relatively high temperatures that are required to process these resins severely limit their industrial utility.
A need therefore exists for a polyimide resin for use as a matrix resin in a fiber-reinforced composite that possesses properties rendering it suitable for processing by RTM and RI methods at reduced processing temperatures.
It is therefore a primary object of the present invention to provide such a resin.
More particularly, it is an object of the present invention to provide polyimide resins that may be processed at reduced processing temperatures, and that exhibit melting at temperatures of less than about 200° C. and melt viscosities at 200° C. of less than about 3000 centipoise.
It is another object of the present invention to provide a process for synthesizing such RTM and RI processable polyimide resins.
It is a further object to provide a fiber-reinforced composite material that employs such a polyimide resin as the matrix resin and that has good heat resistance and mechanical properties.